The present application relates generally to a control system for a machine with a dual path electronically controlled hydrostatic transmission having variable displacement pumps and/or variable displacement motors.
One type of machine with a dual path electronically controlled hydrostatic transmission or a dual path electro-hydraulic transmission that is used for the propulsion and steering can be a crawler-tractor such as a bulldozer. “Bulldozers” or “dozers,” as those terms may be used herein, refer to crawler-tractors that are equipped with a blade for scraping the ground or pushing material along the ground. The blade is pivotally connected to the crawler-tractor chassis such that it can pivot up and down. Blade controls are provided to the operator in the cab of the vehicle to permit the operator to raise and lower the blade with respect to the chassis of the crawler-tractor. One of the most common uses for blades on bulldozers is to level or otherwise contour the ground for the construction of houses, buildings, parking lots, and roads.
When an operator performs a rapid acceleration or deceleration of the machine in a forward or reverse direction, the result is a sudden increase or decrease of transmission oil flow. In response to the sudden increase or decrease of transmission oil flow, a resultant increase in torque is translated to the engine. The sudden increase in torque can cause a reduction in the engine speed resulting in a sudden drop in machine speed, which is undesirable to the operator. In other words, the generation of a high torque can exceed engine torque limits and cause engine lug down.
When executing a turn for a machine with a dual path electronically controlled hydrostatic transmission, the sliding friction between the wheels or track drives and the ground increase as the turning radius of the machine decreases. In response to this increased friction, an increase in torque is translated to the engine of the machine by way of increased driveline pressures. The transmission of the machine has a finite pressure limit governed by system relief valves. When the transmission system pressure increases to the relief valve setting, the machine may stall. In order to prevent stalling as a turn is executed, the motor displacement in the transmission must increase to effectively decrease the pressure. The increase in motor displacement causes the machine speed to decrease leading to operator dissatisfaction.
Further, the machine can be used for heavy-duty operations such as push-pull operations, lifting operations, digging operations, etc. Performing these heavy-duty operations can stall the engine by creating a torque load that exceeds the available torque of the engine.
Machines with a dual path electronically controlled hydrostatic transmission, such as the bulldozer or other crawler-tractor, can have an independent pump and motor to drive each of the left side and the right side of the machine's driveline. At times, the pump and/or motor displacement on each side of the driveline can vary in an unpredictable manner due to various reasons, including manufacturing tolerances, hydraulic driveline inefficiencies, “wear and tear” of the pump and motor components, loading conditions, etc., that can result in non-straight, i.e., curved or angled, movement of the machine when the machine is commanded or steered in a straight direction.
Therefore, what is needed is a control system to control the variable pumps and variable motors used in a dual path electronically controlled hydrostatic transmission in order to maintain desired performance of the machine while performing various machine operations.